X-ray imaging is widely used to image internal organs for diagnostic purposes and to assist health practitioners during therapeutic interventions. For example, this technique has found many applications in cardiology including intra ventricular interventional cardiac procedures such as Direct Myocardial Revascularization and electro physiological mapping and ablation.
In order to extract useful structural information from X-ray irradiation of an object, the projections data must often be extensively processed using image reconstruction techniques. Such image reconstruction techniques are well-known in the art and include for example, algorithms known as ART (Algebraic Reconstruction Techniques). A good introduction to these concepts can be found in Principles of Computerized Tomographic Imaging, by Kak and Slaney, IEEE Press, 1988. Using these techniques, X-ray tomographic images can be obtained by irradiating the object to be examined from a multiplicity of directions and processing the information from each of the projections to reconstruct the image of the object.
Image reconstructions based on ART often require many projections from different angles (often more than 100) to obtain good quality images. As a result, acquisition times are long and necessitate the immobilization of the patient for prolonged periods to minimize positional errors. Furthermore, the long acquisition times may be prohibitive for emergency procedures and may reduce the availability of the apparatus.
Methods have been devised to reduce the number of projections required to obtain satisfactory images. One such method is disclosed in U.S. Pat. No. 4,672,651 wherein two X-ray projections of an object are obtained from which an initial image of the object is corrected by a relaxative procedure. More specifically, the shape of the object is modified or corrected in accordance with individual data values in the projection images for the two directions, thereby producing a corrected shape. Eiho et al. (Japanese J. of Soc. of Instrument and Control Engineers Vol. 19 No.10, Pages 923-936, 1980) disclose a similar method for image reconstruction using two projections. However, these methods are generally limited to ellipsoid objects and require extensive computational time.
A further method has been described in U.S. Pat. No. 5,442,672 that uses two X-ray projections to tomographically reconstruct images of objects in baggages. The method is based on multiplicative algebraic reconstruction techniques (MART) wherein successive slices of the object are projected. This method requires that the value of each pixel in the image intercepted by a given X-ray, be adjusted to the average density of the object along the path of the X-ray. However, the method is somewhat limited by the heterogony of the object""s densities which have to be estimated. Accordingly, a number of constraints have to be applied pertaining to the characteristics of the objects such as density and spacial continuity for example. Furthermore, the acquisition of several slices within each projection and their processing for image reconstruction may be time consuming.
In view of the state of the art, it would be desirable to provide a method requiring few projections and rapid computational calculations to effect the correction of the projections data.
The present invention relates to a method for reconstructing the image of an object using at least two X-ray projections.
In one aspect of the present invention there is provided a method for reconstructing the image of an object using known iterative reconstruction techniques. A second aspect of the instant invention is to obtain good quality image reconstructions using a limited number of projections. Further aspects of the method of the instant invention will be described in the detailed description of the invention which follows.
According to the present invention, there is provided a method for reconstructing the image of an object substantially uniformly filled with an X-ray contrast agent using iterative reconstruction techniques in which the absorbance value of the pixels above a pre-selected threshold value are set to zero and the remaining pixels are set to the absorbance value corresponding to the concentration of the X-ray contrast agent within the object.
Thus, according to the present invention there is provided a method for reconstructing an image of an object using at least two single plane X-ray projections, the method comprising the steps of; substantially uniformly filling the object with an X-ray contrast agent; irradiating said contrast agent-filled object with X-rays from at least two directions; detecting said X-rays transmitted through said contrast agent-filled object to define at least two live X-ray projection absorbance profiles; providing a first image to be corrected; updating said image by setting the absorbance value to 0 for all picture elements having an absorbance below a preselected threshold value and by setting all other picture elements to the absorbance value corresponding to the concentration of the X-ray contrast agent within the object; forward projecting the updated image into said at least 2 planes of projection along said at least two directions to obtain X-ray absorbance profiles of said image to be corrected; calculating the difference between the absorbance profiles of the object and of the image to be corrected; correcting the updated image using iterative reconstruction techniques; repeating the correction until the difference between the current absorbance profiles and the object absorbance profiles is less than a selected value; displaying the image.
According to the present invention is also provided a method for reconstructing an image of an object using at least two single plane X-ray projections, the method comprising the steps of; irradiating said object with X-rays from at least two directions; detecting said X-rays transmitted through said object to define at least two projection mask X-ray absorbance profiles; substantially uniformly filling the object with an X-ray contrast agent; irradiating said contrast agent-filled object with X-rays from at least two directions; detecting said X-rays transmitted through said contrast agent-filled object to define at least two live X-ray projection absorbance profiles; subtracting said mask profiles from said live profiles to define a final absorbance profile for each projection; providing a first image to be corrected; updating said image by setting the absorbance value to 0 for all picture elements having an absorbance below a preselected threshold value and by setting all other picture elements to the absorbance value corresponding to the concentration of the X-ray contrast agent within the object; forward projecting the updated image into said at least 2 planes of projection along said at least two directions to obtain X-ray absorbance profiles of said image to be corrected; calculating the difference between the absorbance profiles of the object and of the image to be corrected; correcting the updated image using iterative reconstruction techniques; repeating the correction until the difference between the current absorbance profiles and the object absorbance profiles is less than a selected value; displaying the image.
In an embodiment of the present invention a method is provided to reconstruct images of ellipsoid objects using 2 projections. In a further embodiment, a method is provided to reconstruct the image of non-ellipsoid objects using 3 or more projections.